Is 4GB Ram sufficient for a 64 bit OS

Turns out that the installed RAM was in an unsupported config -- On this machine, if you install 2GB chips, they have to be installed in triple, as referenced here (pdf).

Its interesting to note that Windows saw the memory, but it was rendered not usable and marked as "hardware reserved".

Well, solving this is black magic. Try with swapping slots and sticks. Another thing could be order in which the sticks were inserted. All this shouldn't really have any effect on amount of detected RAM, but I've seen some temperamental motherboards that will detect RAM only if certain sticks are in certain slots and if the sticks are inserted in correct order.

Basically, try experimenting with different arrangement. Also, get some memory test utility, like memtest86+ form http://www.memtest.org/ and use it between attempts to see if all sticks are good. It could be that some are damaged by static electricity.

About the RAM reserved by system: Unless you have some outrageous integrated graphics card, it shouldn't take more than few MiBs at most.

Also, check if there is some known incompatibility with your motherboard, your RAM and operating system. For example, nForce 2 motherboards won't work (or at all) well with vista or 7 if they have more than 2 GiBs of RAM.